Cellular radio systems use base station radio transceivers that are mounted, typically on buildings or on towers, to supply radio signals in their service areas. In the classical analog systems, and in most digital systems, a number of communications channels having separate frequencies are used at each base station, and these channels are reused at other base stations that are spaced apart by a minimum reuse distance. To improve the efficiency of this reuse, sectorized antennas are used to divide a cell into 3 or more sectors. By dividing the cell in this way, the typical analog cellular system can reduce its reuse pattern from 12 cells in a cluster down to 7 cells per cluster, a cluster being the number of cells with unique frequency/channel sets before reuse is allowed. Thus, by reducing the cluster size, more channels are available at each cell, which yields an improvement in capacity.
However, by dividing an omni pattern into a sectored pattern, the transmit and receive antennas in a given sector will only see a fraction of the sector. Because of this, the frequency assignments must be divided among the sectors. In other words, given sectors 1, 2 and 3, the channels in sector 1 cannot, in general, be used to cover sectors 2 or 3 since the antennas point in different directions. Thus, there are now separate frequency groups for each sector if the cell is sectorized.
FIG. 1 is a prior art table of the Erlang B distribution. This is given here as background information describing the effect of subdividing the typical 3 to 6 sector base stations into a much larger number of sectors. Typically for AMPS (Advanced Mobile Phone Service) cellular, a 3 sector 7-cell/cluster system could be assumed to have up to 19 channels per sector with an evenly distributed frequency assignment plan. By adding additional sectors, the number of frequencies per sector generally goes down, even with the cluster size being reduced. For a 6 sector 4-cell/cluster system, the number of channels is reduced to 16 per sector. Since there are currently proposals suggesting numbers of sectors up to 24 per cell, the number of channels per sector will be significantly less when they are equally divided as in a standard reuse plan. FIG. 1 shows the effect of having fewer servers in a channel set. The result is a loss in trunking efficiency, i.e., the average number of servers that can be supported decreases faster than the number of channels.
FIG. 2 is a prior art diagram illustrating a standard 3 sector pattern for a base station coverage area 5 in which each sectors 10, 20, 30 are each served by a 120 degree antenna pattern 11, 21, 31. These sectors 10, 20, 30 are typically implemented by using three separate antennas mounted on a building or tower.
The actual capacity at each cell or sector, if the cell is divided into sectors, is a function of the number of frequencies that are available in each frequency group. Since user traffic is not a uniform process, i.e., users tend to arrive in a manner typical of a Poisson process, and with exponential call durations, the capacity is normally specified as an Erlang B capacity at a given blocking rate. For example, if a there were 19 frequencies available in a given sector, then according to the Erlang B table, an average of 12.5 users can be served and this will give a blocking rate of 2%, which means that 2% of the time, more than 19 users will try to obtain service. Thus, a channel set of 19 frequencies would support 12.5 Erlangs of traffic at the given grade of service (GOS) or blocking percentage.
Sectorization typically improves the range and capacity of a cell--the range due to the added antenna gain that is inherent with sectorized antennas, and the capacity due to improvements in reuse. Therefore, it should be of benefit to increase the number of sectors within a cell to some very large number. The typical numbers of sectors used in the world today are 3 to 6. The problem with using more than 6 sectors, however, is the effect of dividing the frequency group into smaller and smaller groups. It is typically desirable to keep a small, say 3-sector, group of frequencies, while being able to switch the frequencies within this group for use via a narrower beam within this channel set. However this would require an enormous amount of hardware to switch say 20 frequencies to any number of beams within a 120 degree sector. Thus it would be desirable to simply divide the frequency group into the number of beams that are to be supported. This produces a limitation in capacity however, since the number of Erlangs decreases rapidly. For example, with 8 channels per beam, and a 2% GOS, only 2.5 Erlangs is available in each beam.
There remains, therefore, a need for an improved method of assigning channels to subscribers in a narrow beam communication system.